The phenotype of a living thing is its appearance, meaning how it looks in physical traits. The dominant allele, (the capital letter) means that even when paired with a recessive allele, (the lowercase letter) the phenotype will represent the dominant trait. This means that the plant with the genotype RrTt has a phenotype of red leaves, and tall stems. Bonus: An allele that consists of a dominant gene, and a recessive gene of the same trait, is called a heterozygous allele.
It is called a double circulatory system because blood passes through the heart twice per circuit. The right pump sends deoxygenated blood to the lungs where it becomes oxygenated and returns back to the heart. The left pump sends the newly oxygenated blood around the body.
